This application claims priority under 35 U.S.C. § 119(a) on Japanese Patent Application No. 2005-256725 filed in Japan on Sep. 5, 2005, the entire contents of which are hereby incorporated by reference.
The present invention relates to optical coupling devices that transmit a signal with a light emitting element and a light receiving element, and more specifically, optical coupling devices in which by having a plurality of input-side light emitting elements, high speed operation and a long lifespan are possible, and to electronic apparatuses in which such an optical coupling device is used.
In an optical coupling device, in order to realize high speed response, high speed response of an input-side LED (light emitting diode) is essential. However, because an internal capacity C is present in the LED, high speed response is limited.
FIGS. 5(a) and 5(b) show an ordinary light output waveform (waveform of rising and falling of the LED) when a driving current is applied to the LED. When a high speed pulse driving current as shown in FIG. 5(a) is applied, the actual waveform of the LED is a waveform like that shown in FIG. 5(b). That is, delay times tr and tf respectively occur in the rising and falling of the LED. In this case, for example if an AlGaAs LED, data transfer of 10 MB/s is not a problem. However, when driving at a higher speed than that, it is necessary to control the LED by providing a circuit for controlling the LED on the input side (the sending side).
FIGS. 6(a) to 6(c) show a conventional current application method that drives an LED at high speed (for example, see JP H11-74567A).
In this type of current application method, for an input waveform as shown in FIG. 6(a), with a technique known as peaking, a current I3 that is greater than a driving current that corresponds to a set light output is applied only during the rise. Thus, the rise time tr is made a short time. Also, in this method, with a technique known as a backshoot, the LED is temporarily given a reverse bias during the rise. Thus, the fall time tf is made a short time. Moreover, with this method, in order to make a change in the capacity of the LED as small as possible, a constant current I1 is applied from an LED driving circuit at a level that does not exceed a threshold current of a light receiving element even when the signal is off. Thus, an even faster response is made possible in the optical coupling device. However, by increasing the value of the fixed current I1, the power consumed when the input signal is off increases.
In the technology described in JP H11-74567A, a control method is disclosed for allowing the LED to respond quickly, but in for example an optical coupling device such as a photocoupler, constant driving of the LED results in an increase in power consumption, so this method was not applied in the conventional technology.
That is, in an optical coupling device such as a photo-coupler, when, in order to quickly drive on/off, driving current is constantly applied to an LED on the input side (sending side) even when the signal is off, there is the problem that power consumption increases. Also, by constantly driving the LED, deterioration of the LED itself occurs more quickly, and product defects are generated by a decrease in the amount of light, leading to a shorter product lifespan.